This invention relates to a numerical control (NC) system for a machine tool and in particular to a numerical control system capable of effecting the numerical control of the machine tool by a computer or a wire-connected controller and capable of effecting the numerical control of the machine tool by directly controlling a servo motor for moving the tool.
In a conventional NC system an interpolating device in a hardware performs an interpolating function such as a straight line interpolation, circular arc interpolation etc. A now available NC system is required to produce a maximum interpolating speed of more than 200,000 unit instruction pulses per second and in consequence a high speed interpolating device is required to meet the requirment, making it difficult to its realization. If on the other hand an interpolating function per unit pulse is executed using a computer, the computer must calculate and transfer an interpolating data per 5 .mu.sec, thus largely exceeding the processing capablity of the general-purpose computer and making it impossible for the computer to have such a processing speed.
To attain an excellent servo chracteristic a speed control loop is provided in which a motor speed voltage is fed back through a minor loop by a tachometer generator connected to the motor. However, it is desirable that the tachometer generator be disposed from the standpoint of economy and reliability.